1. Field of the Invention
The present invention relates to a glass micro-optical element such as a grating, a micro-Fresnel lens or a micro-lens array, a method for manufacturing a glass micro-optical element, a die used for forming a micro-optical element and a manufacturing method therefor.
2. Description of the Prior Art
In the prior art, when a grating is produced, a thermo-plastic resin is processed directly to form a grating (refer to Japanese Patent laid open Publication 54-110,857 and Japanese Patent Publication 60-25,761). However, such a resin grating has a disadvantage that the grating changes its volume or its shape with changes in environment such as temperature or humidity. Then, the accuracy of the grating is reduced. Further, because the strength of the resin is low, the surface is liable to be damaged. Thus, a very accurate and reliable grating cannot be produced with a thermo-plastic resin.
On the other hand, a glass grating has advantages that its durability is good, that it is not easily damaged on the surface, and that its accuracy is not deteriorated due to the environment. A method is proposed wherein a glass grating is formed directly on the glass surface by dry etching (Japanese Patent laid open Publication 55-57,807). However, it takes a long time to produce a grating using this method, and the mass production of the gratings of the same shape is impossible.
Recently, mass production by press-molding of glass was proposed for an optical glass element such as an asymmetrical glass lens. If a very accurate optical glass element is produced with the press-molding method, the shape of the optical glass element is required to be reproduced well. Therefore, a material of a die used for the press-molding is required to be inactive for glass at high temperatures, to be sufficiently hard at a surface portion in contact with glass on forming the glass, to be not easily damaged by friction and the like, to be hard to deform plasticly or hard to grow crystal grains at the surface portion at high temperatures, to be superior in heat shock resistance and to be superior on the workability for very accurate forming.
The materials for a die which satisfy these conditions to some extent include SiC and Si.sub.3 N.sub.4 (refer to Japanese Patent laid open Publication 52-45,613). If SiC or Si.sub.3 N.sub.4 is used as a die material, the problem exists that it is very difficult to form a die of a desired shape because the material is very hard. Further, the material is reactive with glass at high temperatures, and glass adheres to the die after repetitive forming with the press so that an accurate grating cannot be formed.
Further, a die has been recently proposed wherein a base material of a hard metal is coated with a platinum group alloy thin film (refer to Japanese Patent laid open Publication 60-246,230). The die can be used for grinding, but it cannot be used to produce a fine shape accurately. Thus, the die cannot be used as a die for a grating.
As to micro-optical elements other than gratings, a die for press-molding glass to form a reliable and accurate micro-optical element also has not yet been produced.